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sg                          Worm Wheel Pattern.

what is meant: where A is the pattern, n the rore lix, ami
c the mould, the object being a 'dummy' gland for a strain
cylinder.

Referring again to the Worm Wheel in Fig. i2,tlK-- inrtti^i
of making the pattern will be understood by the help of H&s. 7^
71, and 72. It may be built in the way shewn for the pulley in
Fig. 63, and, after being turned on the rim, blocks of hard wuocl
are fitted on each half of the pattern, and glued in tin; nunm-i-
suggested at D (Fig. 72).

The outside surface of these blocks is now turned *< as to
give a solid rim of wood, from which the teeth are to be rut.    To
do this a stud A, Fig. 71 (on the table of a wood-working marhincj
is fixed at the angle of the worm thread, and the wheel pattern stt
upon   it, so that it can be rotated carefully the amount  of the
pitch., by gearing, much on the same principle as in a moulding
machine.    A revolving cutter B, driven at from 2,000 to 3,000
revolutions per minute, is advanced to the pattern, and ruts
the space between the teeth; the diameter*/of this cutler
be the same as that of the worm.    When this operation Is oint
pleted, the wheel is removed and placed on stud c, Fig, j 2.    Tin*
wrought-iron worm   intended to work with  the  casting,  l*t*ifig
marked with red ochre, is now advanced, together with itn wcwicl
bearings, to gear with the pattern, and the worm is rotated ;
wherever a little mark is left by contact of the worm, the            is

gouged away until a perfectly correct fit is obtained.

Spur Wheels too small to make by machine moulding
have their teeth formed by the revolving cutter shewn at it,        of
course, in that case, the axes of wheel and cutter are at right

For machine-moulded wheels, either spur or bevel, the
is to be supplied with a block of pine with two teeth dovetailed in
harder wood, as in Fig. 720 (the machine is shewn at Fig. 46),

In both sketches the direction of withdrawal is shewn by the
arrows, and it will be seen that, although the bevel teeth
without difficulty, there would be some risk of the sand               to

the pattern in the case of the spur teeth, which are made
perpendicular and without taper.    To avoid such an                 a

finger bit A is provided, which, fitting in the hollow               the

two teeth, presses down the sand as the block is lifted